Documentation rewording

Signed-off-by: TRodziewicz <tomasz.rodziewicz@mobica.com>

library/timing.c

@@ -83,6 +83,25 @@ unsigned long mbedtls_timing_get_timer( struct mbedtls_timing_hr_time *val, int
 
 #else /* _WIN32 && !EFIX64 && !EFI32 */
 
+/**
+ * \brief          Return the elapsed time in milliseconds
+ *
+ * \warning        May change without notice
+ *
+ * \param val      points to a timer structure
+ * \param reset    If 0, query the elapsed time. Otherwise (re)start the timer.
+ *
+ * \return         Elapsed time since the previous reset in ms. When
+ *                 restarting, this is always 0.
+ *
+ * \note           To initialize a timer, call this function with reset=1.
+ *
+ *                 Determining the elapsed time and resetting the timer is not
+ *                 atomic on all platforms, so after the sequence
+ *                 `{ get_timer(1); ...; time1 = get_timer(1); ...; time2 =
+ *                 get_timer(0) }` the value time1+time2 is only approximately
+ *                 the delay since the first reset.
+ */
 unsigned long mbedtls_timing_get_timer( struct mbedtls_timing_hr_time *val, int reset )
 {
     struct _hr_time *t = (struct _hr_time *) val;